SECTION XI CRYSTALLIZATION
BY ARTHUR GIVEN*
Crystallization is one of the oldest and also one of the most important processes applied to chemical products. It is one of the oldest, indeed perhaps the second oldest, since it can in many cases be brought about by the simplest of means, concentration due to natural evaporation. Concentration is probably the oldest chemical process known to man since it occurs naturally in the cases of natural solutions, e.g., sea water in shallow rock pools, or in drying the juices of ripened fruits. Crystallization must be regarded as the second oldest chemical process as being the direct result of concentration. It may be contended that solution should be given first place for age, but it seems clear that this could only have been a man-controlled process after there were crystals prepared from natural solutions.
The term "crystallization" as used in this article is intended to cover not only the production of chemical compounds in the form of actual measurable crystals, but also the production from solution of crystallizable compounds desired, in any solid form, either crystalline or amorphous.
The great importance of crystallization lies in the three chief things which it accomplishes:
1. It produces the substance in a solid form, thus making it much simpler and easier to transport it and to preserve it.
2. It gives a great decrease in weight and usually in bulk, which further increases the advantages under (1).
3. It produces a material of a greater, usually much greater, degree of purity than the solution from which derived.
A large majority of chemical products as such, whether natural or synthetic, are first produced as solutions of greater or less purity and concentration. In many cases where these solutions are of sufficient purity, i.e. contain besides the solvent either no impurities which will interfere with later use, or only small amounts of such impurities, crystallization offers no advantages. A few cases may be cited:
(a) When the substance cannot be produced in the desired form or composition in the solid phase.
(o) When the substance is to be used as a solution in close proximity to the point of production.
(c) When it is not readily usable in the solid form.
It is, however, far more usual for chemical compounds which are solids in a pure state at normal temperatures to be finally manufactured, handled and sold in some form of crystalline condition.
Since crystallization invariably means more or less purification, even though by no more than the removal of the solvent, the ideas and processes of crystallization and purification are inextricably interwoven and it is impossible to consider crystallization
1 Formerly 1st. Lt., Ord. Dept., U. S. A.; Late Chemical JCngiiieer, Picatinny Arsenal, Dover, New Jersey. Deceased.